MPLS Layer-2 virtual private network (MPLS L2VPN) implements L2VPN across an MPLS backbone by encapsulating user packets in MPLS labels. Virtual private LAN service (VPLS) and virtual private wire service (VPWS) are two approaches to MPLS L2VPN.
Both VPLS and VPWS fit into the PW emulation edge-to-edge (PWE3) architecture. In the PWE3 architecture, a PW is a dedicated virtual connection from PE to PE, which is achieved by using encapsulation technology. In VPLS/VPWS applications, VPLS/VPWS access is provided by PEs. Multiple PWs can share one physical link while being independent of each other. Each PW contains two unidirectional virtual circuits (VCs) in different directions. After the two VCs are established, a bidirectional PW is established.
Currently, a PW can be established manually, through Martini MPLS L2VPN, or through Kompella MPLS L2VPN. The Martini MPLS L2VPN is achieved by extending the Label Distribution Protocol (LDP) while the Kompella MPLS L2VPN is achieved by extending the Border Gateway Protocol (BGP). Among the three approaches, Martini MPLS L2VPN has gained wide application because it avoids the limitation of manual setup in network size and the deficiency of Kompella MPLS L2VPN in deployment flexibility.
To guide Martini signaling implementations, the IETF drafted RFC 4477. According to the RFC, The PEs in the VPLS or VPWS must first establish LDP neighborhoods and then send Label Mapping messages to each other to advertise local PW information.
In a Label Mapping message, each PW entry contains a PW ID, a PW label, and a set of interface parameters. RFC 4447 defines two types of type-length-value (TLV) attributes for PW identification: PWid FEC and Generalized PWid FEC. The PWid FEC uniquely identifies a PW by using the combination of PW type and PW ID (LDP neighborhood information in the messages may be required in addition), and identifies a PW group by using the group ID. Generalized PWid FEC uniquely identifies a PW by using the combination of AGI, SAII and TAII, and identifies a PW group by using the PW Grouping ID TLV.
The methods used by the two FECs to identify groups of PWs are called wildcard identification. The Group ID and the PW Grouping ID TLV used by the two FECs are called PW group identifiers. PW grouping is done by the sender of Label Mapping messages and the transmitted label mappings already carry the PW identifiers of PWs. The PE that receives the label mappings saves the Group ID or PW Grouping ID TLV, that is, the PW group identifier, for each PW. When the receiving PE device receives a wildcard message, it can use the group identifier of PW to identify whether a PW belongs to the PW group affected by the message.
After a PW is established, the PEs at the ends of the PW can transmit packets on it. Because the PW may fail due to the attachment circuit (AC) failure of VPWS or public network LSP failure of VPLS or any other reason, the PEs need to transmit PW unreachable messages to notify the remote PEs of the PW failure.
As described in RFC 4447, the PW unreachable message must contain the PW ID for identifying the unreachable PW. To improve transmission efficiency, wildcard identification, that is, the PW group identifier, can be used in PW unreachable messages to notify the remote endpoint of affected PWs in groups.
In present application, PW grouping is achieved by using the Group ID or PW Grouping ID TLV in the PW identifier, and only one PW grouping method can be used. This can hardly cover various failure causes and constrains the use of wildcard messaging.
Suppose PW groups are divided by physical ports connected to ACs. When a physical port fails causing the failure of all PWs on it, the PE can use a wildcard message to notify its remote peer of the PW failure. When a public network LSP fails causing the failure of all PWs on it, the PE cannot do this. Rather, the PE has to send one message to the remote peer for each failed PW.
If PW groups are divided by public network LSPs, the PE can use a wildcard message to notify its remote peer of the PW failures caused by a public network LSP failure. However, to notify of the PW failures caused by a physical port failure, the PE has to send a PW unreachable message for each PW.
Thus, single-method based PW grouping limits the use of wildcard messages, decreasing efficiency. In cases where a PE has to send a message for each unreachable PW, the delay of unreachable PW advertisement is increased and system resources are wasted.